Implement ticket 057: fix rate() line charts for sparse/bursty occurrences

A rate() series over a metric whose occurrences are confined to a short
burst within a much wider queried range (e.g. sum by (code)
(rate(KeyPressed[5m])) against a ~10h log where all key presses happen
in the first 10s) rendered as broken: the line simply stopped at the
last real occurrence, and the chart's x-axis auto-scaled to the tiny
span of actual data points rather than the queried range, silently
narrowing a 10-hour view down to ~10 seconds with no indication anything
was off.

Backend (src/engine.rs): windowed_rate_series now appends a synthetic
boundary point at start_ms/end_ms (computed via the existing
rate::rate_at against the original occurrence points, so a boundary
inside a still-warm window gets its true partial value rather than a
hardcoded 0) whenever the real series doesn't already reach that edge.
Wired into both the bare rate() and sum()/avg(rate()) evaluate_promql
branches, which already had start_ms/end_ms in scope.

Frontend (PanelChart.tsx/Dashboards.tsx): the line chart's XAxis now
uses an explicit numeric domain pinned to the panel's queried range
(passed down from Dashboards.tsx's PanelPreview) instead of recharts'
default auto-scale-to-data-extent, so sparse data can no longer make
the axis look narrower than what was actually queried.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/Dashboards.tsx

@@ -96,7 +96,7 @@ function PanelPreview({ panel, range }: { panel: Panel; range: TimeRange | null
   if (loading && series.length === 0) {
     return <div className="flex h-48 items-center justify-center text-xs text-text-faint">Loading…</div>
   }
-  return <PanelChart series={series} chartType={panel.chart_type} />
+  return <PanelChart series={series} chartType={panel.chart_type} range={range} />
 }
 
 function AutosaveStatus({ status, error }: { status: SaveStatus; error: string | null }) {